Owens Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Owens Observatory clear sky chart from 2005-11-06 to 2024-03-18.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	14% 6% 2 5% 5% 3 5% 5% 6% 8% 42%	79058
	Night	14% 6% 2 5% 5% 3 4 4 5% 7% 44%	80724
Transparency	Day	39% 7% 12% 19% 18% 4	78781
	Night	38% 6% 11% 19% 20% 6%	80520
Seeing	Day	16% 9% 21% 23% 20% 11%	13961
	Night	16% 8% 23% 29% 21% 4	24108

By Month

Percentage Cloud Cover SE (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	19% 10% 3 6% 4 2 4 4 5% 5% 37%	2836
	11th to 20th	Day	14% 6% 2 5% 4 2 4 4 4 5% 49%	2840
	21st to end	Day	27% 8% 2 4 4 2 3 3 3 5% 40%	3234
February	1st to 10th	Day	17% 8% 2 6% 5% 3 4 5% 5% 6% 39%	1900
	11th to 20th	Day	22% 5% 3 4 4 2 4 4 4 6% 42%	1848
	21st to end	Day	22% 6% 3 5% 5% 2 4 4 4 6% 39%	1570
March	1st to 10th	Day	16% 6% 3 5% 4 2 4 3 5% 7% 44%	1956
	11th to 20th	Day	19% 6% 2 6% 5% 3 4 5% 5% 6% 39%	2058
	21st to end	Day	16% 4 3 5% 5% 3 5% 5% 6% 8% 39%	2178
April	1st to 10th	Day	12% 4 2 5% 4 3 4 4 7% 7% 46%	2029
	11th to 20th	Day	13% 5% 3 5% 5% 3 4 4 5% 7% 47%	2134
	21st to end	Day	10% 3 2 5% 4 3 4 5% 5% 8% 51%	2307
May	1st to 10th	Day	9% 5% 3 5% 4 3 4 4 5% 8% 50%	2340
	11th to 20th	Day	11% 5% 2 5% 6% 3 5% 6% 6% 9% 41%	2301
	21st to end	Day	10% 5% 2 5% 5% 3 4 6% 7% 10% 43%	2531
June	1st to 10th	Day	10% 6% 3 6% 6% 3 5% 6% 7% 12% 36%	2340
	11th to 20th	Day	5% 4 2 5% 4 3 7% 6% 9% 12% 43%	2340
	21st to end	Day	8% 4 2 6% 6% 4 6% 7% 8% 11% 39%	2340
July	1st to 10th	Day	9% 5% 3 6% 7% 5% 8% 7% 9% 11% 30%	2340
	11th to 20th	Day	10% 5% 3 6% 6% 5% 7% 8% 9% 12% 30%	2340
	21st to end	Day	6% 4 3 6% 8% 5% 7% 9% 10% 11% 30%	2547
August	1st to 10th	Day	9% 4 2 6% 7% 6% 8% 8% 9% 11% 30%	2327
	11th to 20th	Day	13% 6% 2 6% 7% 4 6% 7% 8% 9% 32%	2168
	21st to end	Day	13% 6% 3 8% 8% 5% 6% 6% 7% 10% 29%	2376
September	1st to 10th	Day	11% 6% 2 6% 6% 3 5% 6% 7% 8% 40%	1985
	11th to 20th	Day	15% 5% 3 5% 5% 3 5% 5% 6% 7% 42%	1980
	21st to end	Day	14% 5% 2 5% 4 2 4 4 4 7% 49%	1980
October	1st to 10th	Day	13% 5% 1 4 3 3 3 3 4 5% 56%	1980
	11th to 20th	Day	11% 5% 2 4 3 2 3 4 6% 7% 54%	1878
	21st to end	Day	14% 5% 2 4 4 2 3 3 4 5% 54%	1980
November	1st to 10th	Day	13% 4 2 3 3 2 3 3 4 5% 57%	1688
	11th to 20th	Day	14% 5% 2 4 3 3 4 4 5% 5% 53%	1710
	21st to end	Day	15% 5% 3 5% 5% 2 4 4 5% 7% 46%	1710
December	1st to 10th	Day	22% 8% 2 5% 5% 2 4 4 5% 6% 38%	1710
	11th to 20th	Day	17% 7% 3 5% 5% 3 4 4 4 5% 43%	2153
	21st to end	Day	23% 6% 3 5% 4 3 4 4 5% 7% 35%	3124
January	1st to 10th	Night	19% 7% 3 4 4 2 4 4 5% 6% 41%	4109
	11th to 20th	Night	14% 7% 3 5% 4 2 4 4 4 6% 45%	4120
	21st to end	Night	26% 8% 3 5% 4 2 3 3 3 4 39%	4406
February	1st to 10th	Night	16% 6% 3 5% 5% 3 4 4 5% 6% 44%	2280
	11th to 20th	Night	22% 6% 3 5% 4 3 4 3 4 4 43%	2218
	21st to end	Night	22% 6% 2 6% 5% 2 4 4 5% 6% 39%	1884
March	1st to 10th	Night	16% 6% 3 4 4 3 3 3 4 6% 48%	2223
	11th to 20th	Night	15% 5% 2 5% 4 3 4 5% 6% 8% 42%	2068
	21st to end	Night	13% 6% 2 5% 4 3 5% 4 5% 7% 45%	2178
April	1st to 10th	Night	11% 4 2 4 5% 2 5% 5% 5% 8% 48%	1944
	11th to 20th	Night	15% 4 1 4 3 2 3 4 5% 7% 51%	1775
	21st to end	Night	9% 5% 2 4 5% 3 4 4 5% 9% 52%	1652
May	1st to 10th	Night	9% 4 1 5% 5% 3 4 6% 5% 8% 51%	1620
	11th to 20th	Night	10% 4 3 6% 6% 3 5% 5% 5% 7% 45%	1592
	21st to end	Night	9% 7% 2 5% 5% 3 6% 5% 5% 9% 45%	1744
June	1st to 10th	Night	7% 5% 2 7% 5% 5% 6% 8% 9% 11% 36%	1620
	11th to 20th	Night	5% 4 2 5% 5% 4 5% 6% 7% 12% 44%	1620
	21st to end	Night	6% 4 1 5% 5% 4 5% 6% 7% 12% 43%	1620
July	1st to 10th	Night	8% 5% 3 6% 7% 4 6% 7% 7% 10% 37%	1620
	11th to 20th	Night	6% 5% 2 6% 6% 5% 7% 7% 7% 13% 36%	1620
	21st to end	Night	6% 5% 3 6% 7% 5% 7% 6% 10% 11% 35%	1771
August	1st to 10th	Night	8% 5% 2 6% 6% 3 7% 6% 8% 12% 36%	1611
	11th to 20th	Night	10% 5% 3 7% 5% 4 5% 6% 6% 9% 40%	1729
	21st to end	Night	13% 6% 3 5% 6% 4 6% 6% 6% 9% 35%	1980
September	1st to 10th	Night	10% 5% 2 6% 5% 3 6% 5% 7% 8% 43%	1954
	11th to 20th	Night	14% 5% 2 5% 4 3 4 5% 5% 9% 43%	1980
	21st to end	Night	12% 4 2 5% 4 3 5% 4 5% 8% 48%	1980
October	1st to 10th	Night	12% 4 2 4 3 2 3 3 5% 7% 55%	1980
	11th to 20th	Night	11% 5% 2 4 3 2 3 4 4 6% 56%	2079
	21st to end	Night	13% 4 2 4 4 1 3 4 4 6% 56%	2376
November	1st to 10th	Night	12% 4 2 3 4 2 3 3 3 6% 57%	2387
	11th to 20th	Night	15% 5% 2 3 3 2 3 3 4 6% 54%	2470
	21st to end	Night	16% 4 2 4 4 2 4 5% 5% 6% 47%	2470
December	1st to 10th	Night	18% 6% 3 5% 6% 3 4 4 4 7% 40%	2470
	11th to 20th	Night	18% 7% 2 4 3 2 3 3 4 6% 47%	3042
	21st to end	Night	20% 8% 3 6% 5% 3 4 4 5% 7% 34%	4532

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	48% 4 7% 15% 21% 6%	2837
	11th to 20th	Day	36% 3 9% 22% 23% 6%	2840
	21st to end	Day	48% 2 7% 17% 21% 6%	3234
February	1st to 10th	Day	46% 4 8% 17% 20% 5%	1900
	11th to 20th	Day	45% 4 7% 18% 19% 6%	1858
	21st to end	Day	47% 4 8% 14% 20% 8%	1570
March	1st to 10th	Day	40% 4 9% 17% 20% 9%	1956
	11th to 20th	Day	46% 5% 11% 15% 18% 6%	2058
	21st to end	Day	41% 7% 12% 22% 16% 3	2178
April	1st to 10th	Day	36% 5% 10% 23% 20% 5%	2029
	11th to 20th	Day	38% 5% 8% 19% 23% 7%	2134
	21st to end	Day	30% 6% 12% 29% 21% 3	2307
May	1st to 10th	Day	32% 6% 13% 24% 22% 3	2340
	11th to 20th	Day	37% 10% 15% 23% 14% 1	2305
	21st to end	Day	34% 9% 13% 26% 17% 1	2531
June	1st to 10th	Day	38% 11% 17% 23% 10% 2	2340
	11th to 20th	Day	28% 11% 21% 27% 12% 2	2340
	21st to end	Day	35% 11% 16% 24% 11% 2	2340
July	1st to 10th	Day	41% 13% 19% 17% 8% 2	2275
	11th to 20th	Day	40% 14% 21% 15% 9% 2	2271
	21st to end	Day	39% 15% 18% 20% 7%	2405
August	1st to 10th	Day	40% 15% 19% 21% 5% 1	2314
	11th to 20th	Day	44% 12% 16% 18% 9% 2	2168
	21st to end	Day	46% 11% 14% 17% 10% 2	2376
September	1st to 10th	Day	39% 9% 14% 16% 17% 4	1985
	11th to 20th	Day	39% 8% 11% 17% 22% 2	1980
	21st to end	Day	36% 5% 10% 19% 23% 7%	1980
October	1st to 10th	Day	31% 5% 9% 18% 29% 7%	1980
	11th to 20th	Day	29% 5% 9% 23% 26% 9%	1878
	21st to end	Day	33% 3 7% 18% 29% 10%	1980
November	1st to 10th	Day	31% 3 8% 15% 32% 10%	1688
	11th to 20th	Day	34% 4 8% 17% 28% 10%	1710
	21st to end	Day	38% 5% 7% 19% 25% 6%	1710
December	1st to 10th	Day	46% 6% 6% 20% 16% 6%	1710
	11th to 20th	Day	44% 3 7% 15% 23% 7%	2150
	21st to end	Day	47% 4 11% 17% 18% 3	3124
January	1st to 10th	Night	42% 4 8% 16% 25% 5%	4109
	11th to 20th	Night	40% 4 7% 21% 20% 8%	4120
	21st to end	Night	50% 2 6% 12% 25% 6%	4406
February	1st to 10th	Night	42% 4 9% 17% 20% 8%	2280
	11th to 20th	Night	46% 4 5% 17% 21% 8%	2219
	21st to end	Night	46% 4 8% 16% 17% 9%	1884
March	1st to 10th	Night	39% 3 9% 18% 20% 12%	2223
	11th to 20th	Night	38% 8% 10% 16% 21% 7%	2068
	21st to end	Night	38% 5% 10% 23% 20% 4	2178
April	1st to 10th	Night	34% 5% 11% 20% 24% 7%	1944
	11th to 20th	Night	33% 4 9% 20% 24% 10%	1775
	21st to end	Night	29% 6% 10% 28% 21% 5%	1652
May	1st to 10th	Night	31% 7% 13% 22% 23% 4	1620
	11th to 20th	Night	37% 8% 16% 23% 15% 1	1593
	21st to end	Night	35% 8% 12% 24% 17% 3	1745
June	1st to 10th	Night	37% 12% 17% 20% 12% 2	1620
	11th to 20th	Night	29% 9% 22% 26% 13% 2	1620
	21st to end	Night	31% 11% 20% 21% 14% 3	1620
July	1st to 10th	Night	38% 13% 19% 20% 10% 2	1575
	11th to 20th	Night	35% 15% 22% 16% 9% 3	1566
	21st to end	Night	38% 14% 20% 19% 7% 2	1672
August	1st to 10th	Night	38% 14% 17% 24% 6% 1	1600
	11th to 20th	Night	40% 11% 17% 17% 14% 1	1729
	21st to end	Night	44% 11% 16% 17% 10% 2	1980
September	1st to 10th	Night	37% 9% 13% 19% 17% 4	1954
	11th to 20th	Night	37% 9% 11% 20% 20% 3	1980
	21st to end	Night	34% 7% 11% 17% 22% 9%	1980
October	1st to 10th	Night	30% 5% 12% 18% 28% 7%	1980
	11th to 20th	Night	31% 5% 8% 21% 27% 9%	2079
	21st to end	Night	31% 3 6% 18% 30% 12%	2376
November	1st to 10th	Night	30% 4 7% 18% 28% 13%	2387
	11th to 20th	Night	33% 3 6% 17% 28% 12%	2470
	21st to end	Night	37% 4 7% 19% 25% 8%	2470
December	1st to 10th	Night	44% 5% 8% 19% 19% 5%	2470
	11th to 20th	Night	38% 4 8% 16% 28% 7%	3044
	21st to end	Night	49% 4 8% 19% 17% 3	4532

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	24% 21% 39% 14% 2	270
	11th to 20th	Day	26% 19% 31% 17% 6% 1	270
	21st to end	Day	24% 23% 37% 12% 3 1	297
February	1st to 10th	Day	22% 21% 34% 20% 3	270
	11th to 20th	Day	36% 21% 27% 15% 1	270
	21st to end	Day	31% 18% 36% 13% 2	225
March	1st to 10th	Day	25% 16% 32% 19% 8% 1	326
	11th to 20th	Day	20% 20% 30% 22% 7%	449
	21st to end	Day	18% 16% 34% 23% 8% 1	462
April	1st to 10th	Day	18% 14% 29% 25% 13% 1	420
	11th to 20th	Day	14% 14% 31% 23% 17% 1	412
	21st to end	Day	15% 13% 21% 29% 19% 3	420
May	1st to 10th	Day	13% 10% 30% 26% 15% 6%	423
	11th to 20th	Day	13% 4 22% 28% 21% 13%	447
	21st to end	Day	14% 4 14% 25% 28% 15%	485
June	1st to 10th	Day	12% 3 15% 25% 30% 16%	450
	11th to 20th	Day	7% 3 12% 25% 25% 28%	450
	21st to end	Day	10% 2 14% 27% 27% 19%	450
July	1st to 10th	Day	11% 2 6% 22% 33% 27%	445
	11th to 20th	Day	10% 1 8% 19% 31% 32%	444
	21st to end	Day	6% 1 8% 19% 38% 28%	488
August	1st to 10th	Day	13% 2 12% 22% 26% 25%	446
	11th to 20th	Day	12% 3 7% 21% 38% 20%	439
	21st to end	Day	14% 1 10% 24% 28% 22%	495
September	1st to 10th	Day	11% 4 12% 25% 29% 19%	450
	11th to 20th	Day	15% 4 12% 23% 32% 14%	450
	21st to end	Day	15% 3 16% 24% 29% 12%	450
October	1st to 10th	Day	16% 4 18% 30% 23% 9%	450
	11th to 20th	Day	12% 10% 25% 23% 24% 6%	450
	21st to end	Day	19% 8% 23% 28% 19% 4	495
November	1st to 10th	Day	19% 8% 26% 25% 16% 5%	286
	11th to 20th	Day	17% 18% 34% 21% 8% 2	270
	21st to end	Day	10% 16% 36% 25% 12% 1	270
December	1st to 10th	Day	27% 20% 31% 15% 6% 1	270
	11th to 20th	Day	23% 17% 30% 24% 6% 1	270
	21st to end	Day	32% 13% 32% 17% 5% 1	297
January	1st to 10th	Night	24% 16% 33% 21% 6%	760
	11th to 20th	Night	21% 14% 33% 24% 8%	760
	21st to end	Night	21% 16% 36% 24% 3	836
February	1st to 10th	Night	20% 13% 31% 30% 6%	760
	11th to 20th	Night	25% 14% 34% 25% 2	743
	21st to end	Night	24% 15% 35% 23% 3	628
March	1st to 10th	Night	19% 10% 35% 31% 5%	760
	11th to 20th	Night	16% 12% 35% 26% 10% 1	752
	21st to end	Night	17% 13% 29% 32% 9% 1	792
April	1st to 10th	Night	14% 12% 28% 32% 13% 1	684
	11th to 20th	Night	16% 8% 27% 33% 14% 1	533
	21st to end	Night	10% 7% 26% 35% 20% 1	540
May	1st to 10th	Night	10% 8% 24% 35% 23% 1	540
	11th to 20th	Night	11% 3 23% 32% 26% 5%	531
	21st to end	Night	12% 1 11% 33% 35% 7%	584
June	1st to 10th	Night	9% 1 10% 37% 36% 7%	540
	11th to 20th	Night	6% 1 13% 30% 37% 14%	540
	21st to end	Night	6% 1 10% 35% 38% 10%	540
July	1st to 10th	Night	9% 2 7% 22% 45% 14%	525
	11th to 20th	Night	8% 1 8% 17% 53% 13%	522
	21st to end	Night	7% 1 8% 23% 49% 11%	591
August	1st to 10th	Night	10% 1 9% 23% 46% 11%	537
	11th to 20th	Night	10% 1 9% 26% 46% 8%	534
	21st to end	Night	15% 1 9% 25% 40% 10%	594
September	1st to 10th	Night	11% 2 10% 26% 42% 10%	694
	11th to 20th	Night	16% 1 11% 28% 40% 4	720
	21st to end	Night	14% 3 13% 34% 34% 3	720
October	1st to 10th	Night	13% 3 18% 34% 25% 7%	720
	11th to 20th	Night	14% 5% 24% 32% 21% 3	720
	21st to end	Night	14% 7% 23% 34% 19% 3	792
November	1st to 10th	Night	14% 7% 24% 32% 21% 2	740
	11th to 20th	Night	17% 10% 29% 29% 14% 1	760
	21st to end	Night	19% 9% 28% 30% 14%	760
December	1st to 10th	Night	20% 13% 30% 29% 8% 1	760
	11th to 20th	Night	23% 9% 30% 28% 9% 1	760
	21st to end	Night	27% 13% 26% 29% 6%	836

Page generated at 2024-03-15 14:01:46. Version ac07

Copyright © 2024 Attilla Danko.